The present invention relates generally to optical fibers, and, more particularly, to halide glass optical fibers.
The use of optical fibers for carrying information is of special interest because of several advantages such as security, physical integrity and multiple channel transmission.
In addition to pulse-broadening, transmission loss also limits the distance an optical waveguide can carry light. Transmission loss occurs because of several factors. Impurities in the waveguides absorb some of the transmitted light. In addition, thermal compositional fluctuations, phase separations, inhomogeneities within the waveguide as well as geometric variations in the size of the fiber core scatter a portion of the transmitted light.
If splices must be made because sufficiently long waveguides may not be produced from available performs, these splices further increase transmission loss.
Because fluoride glasses are several orders of magnitude more transparent than conventional silica based glass, fluoride glass has been often mentioned as a material from which to make efficient, low-loss optical fibers. Until now, however, several difficulties have made the use of fluoride glass in optical fibers impractical. Conventional cladding techniques, such as a chemical vapor deposition, cannot be used to make a fluoride glass perform because of the high vapor pressures of fluoride raw materials.
One method of making clad optical fibers is noted in "Preparation of Low-Loss Fluoride Glass Fibers" Electron. Lett , Vol. 18, pp. 170-171 (1982), incorporated herein by reference. According to that process the fluoride cladding melt is poured into a mold which is then upset. The center of the melt flows out and a cylindrical tube is thus formed. Next, the fluoride core melt is poured in to form a preform. The limitations and disadvantages of this process are as follows:
(a) Due to the rapid change in the fluoride glass viscosity with respect to temperature, the cylindrical tube obtained by upsetting the mold is not concentric which leads to undesirable variations in the preform core-clad ratio.
(b) Again due to this high viscosity dependence on temperature, the preparation of long preforms, and therefore long waveguides, is not possible.
(c) And finally, this process appears to be limited to step-index multimode fluoride fibers.
Another method of making such glass fibers is noted in U.S. Pat. 4,519,826 which is incorporated by reference. In this method the cladding glass is poured into a thermally-conductive vertically disposed rotating mold. The mold is then rotated about its vertical axis to allow the cladding glass to coat the bore surface of the mold. The mold is then rapidly changed to a horizontal position while continuing rotation. The centrifugal force from rotation causes the mold to uniformly coat the bore surface of the mold. Rotation is continued until the temperature of the fluoride cladding glass approaches about the temperature of the mold, thus forming a cladding tube. Core glass may melt then be introduced into the cladding tube, thus forming a preform. The preform may then be drawn into an optical fiber.
One possible problem is that the preform must be reheated for the pulling of the clad fiber.